Touch panels have become widely adopted as the input device for a range of electronic products such as smart-phones and tablet devices.
Most high-end portable and handheld electronic devices now include touch panels. These are most often used as part of a touchscreen, i.e., a display and a touch panel that are aligned so that the touch zones of the touch panel correspond with display zones of the display.
The most common user interface for electronic devices with touchscreens is an image on the display, the image having points that appear interactive. More particularly, the device may display a picture of a button, and the user can then interact with the device by touching, pressing or swiping the button with their finger or with a stylus. For example, the user can “press” the button and the touch panel detects the touch (or touches). In response to the detected touch or touches, the electronic device carries out some appropriate function. For example, the electronic device might turn itself off, execute an application, etc.
Although, a number of different technologies can be used to create touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
The most basic method of capacitive sensing for touch panels is the surface capacitive method—also known as self-capacitance—for example as disclosed in U.S. Pat. No. 4,293,734 (Pepper, Oct. 6, 1981). A typical implementation of a surface capacitance type touch panel is illustrated in FIG. 1 and comprises a transparent substrate 10, the surface of which is coated with a conductive material that forms a sensing electrode 11. One or more voltage sources 12 are connected to the sensing electrode, for example at each corner, and are used to generate an electrostatic field above the substrate. When an input object 13 that is electrically conductive—such as a human finger—comes into close proximity to the sensing electrode, a capacitor 14 is dynamically formed between the sensing electrode 11 and the input object 13 and this field is disturbed. The capacitor 14 causes a change in the amount of current drawn from the voltage sources 12 wherein the magnitude of current change is related to the distance between the finger location and the point at which the voltage source is connected to the sensing electrode. Current sensors 15 are provided to measure the current drawn from each voltage source 12 and the location of the touch input event is calculated by comparing the magnitude of the current measured at each source. Although simple in construction and operation, surface capacitive type touch panels are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.
Another well-known method of capacitive sensing applied to touch panels is the projected capacitive method—also known as mutual capacitance. In this method, as shown in FIG. 2, a drive electrode 20 and sense electrode 21 are formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 20 from a voltage source 22. A signal is then generated on the adjacent sense electrode 21 by means of capacitive coupling via the mutual coupling capacitor 23 formed between the drive electrode 20 and sense electrode 21. A current measurement means 24 is connected to the sense electrode 21 and provides a measurement of the size of the mutual coupling capacitor 23. When the input object 13 is brought to close proximity to both electrodes, it forms a first dynamic capacitor to the drive electrode 27 and a second dynamic capacitor to the sense electrode 28. If the input object is connected to ground, as is the case for example of a human finger connected to a human body, the effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement means 24 attached to the sense electrode 21.
As is well-known and disclosed, for example, in U.S. Pat. No. 5,841,078 (Bisset et al, Oct. 30, 1996), by arranging a plurality of drive and sense electrodes in a grid pattern to form an electrode array, this projected capacitance sensing method may be used to form a touch panel device. An advantage of the projected capacitance sensing method over the surface capacitance method is that multiple simultaneous touch input events may be detected.
Inventions have been disclosed in which the touch panel can switch between self-capacitive and projected capacitive modes by means of switches, for example US2014/0078096A1 (Tan et al., March 2014) applies a method to fixed touch panel patterns. The objective of this capability is to use either mode where and when it is more beneficial for object detection.
Moreover, some inventions allow the change of shape or size of the sense and drive electrodes, or their spatial arrangements. For example U.S. Pat. No. 8,054,300 (Apple, November 2011) proposes a method of reconfigurability by means of switches located on the side of the panel or in a separate board.
In many touchscreens the touch panel is a device independent of the display. The touch panel sits on top of the display, and the light generated by the display crosses the touch panel, with an amount of light being absorbed by the touch panel. In more recent implementations, for example U.S. Pat. No. 7,859,521B2 (Apple, December 2010), part of the touch panel is integrated within the display stack, and touch panel and display may share the use of certain structures, such as transparent electrodes. This integration of the touch panel into the display structure seeks to reduce price by simplifying manufacture, as well as reducing the loss of light throughput that occurs when the touch panel is independent of the display and located on top of the display stack.
Another fully integrated touch panel is described in US 2011/0050585 and U.S. Pat. No. 8,390,582B2 (Apple, March 2011 and March 2013 respectively). This patent uses additional signal lines and transistors to switch between display functionality and self-capacitance touch panel functionality, requiring at least three additional transistors per pixel. Display RGB data lines are connected to source/drain transistor terminals, and act as either voltage drive lines or charge sense lines; this prevents the concurrent driving of touch panel and display.